source: rtems/cpukit/score/src/corerwlockrelease.c @ cca2d8ab

/**
 * @file
 *
 * @brief Releases the RWLock
 *
 * @ingroup ScoreRWLock
 */

/*
 *  COPYRIGHT (c) 1989-2006.
 *  On-Line Applications Research Corporation (OAR).
 *
 *  The license and distribution terms for this file may be
 *  found in the file LICENSE in this distribution or at
 *  http://www.rtems.org/license/LICENSE.
 */

#if HAVE_CONFIG_H
#include "config.h"
#endif

#include <rtems/system.h>
#include <rtems/score/corerwlockimpl.h>
#include <rtems/score/threadqimpl.h>
#include <rtems/score/watchdog.h>

CORE_RWLock_Status _CORE_RWLock_Surrender(
  CORE_RWLock_Control *the_rwlock,
  Thread_Control      *executing
)
{
  ISR_Level       level;
  Thread_Control *next;

  /*
   *  If unlocked, then OK to read.
   *  Otherwise, we have to block.
   *  If locked for reading and no waiters, then OK to read.
   *  If any thread is waiting, then we wait.
   */

  _ISR_Disable( level );
    if ( the_rwlock->current_state == CORE_RWLOCK_UNLOCKED){
      _ISR_Enable( level );
      executing->Wait.return_code = CORE_RWLOCK_UNAVAILABLE;
      return CORE_RWLOCK_SUCCESSFUL;
    }
    if ( the_rwlock->current_state == CORE_RWLOCK_LOCKED_FOR_READING ) {
        the_rwlock->number_of_readers -= 1;
        if ( the_rwlock->number_of_readers != 0 ) {
          /* must be unlocked again */
          _ISR_Enable( level );
          return CORE_RWLOCK_SUCCESSFUL;
        }
    }

    /* CORE_RWLOCK_LOCKED_FOR_WRITING or READING with readers */
    executing->Wait.return_code = CORE_RWLOCK_SUCCESSFUL;

    /*
     * Implicitly transition to "unlocked" and find another thread interested
     * in obtaining this rwlock.
     */
    the_rwlock->current_state = CORE_RWLOCK_UNLOCKED;
  _ISR_Enable( level );

  next = _Thread_queue_Dequeue(
    &the_rwlock->Wait_queue,
    CORE_RWLOCK_TQ_OPERATIONS,
    NULL,
    0
  );

  if ( next ) {
    if ( next->Wait.option == CORE_RWLOCK_THREAD_WAITING_FOR_WRITE ) {
      the_rwlock->current_state = CORE_RWLOCK_LOCKED_FOR_WRITING;
      return CORE_RWLOCK_SUCCESSFUL;
    }

    /*
     * Must be CORE_RWLOCK_THREAD_WAITING_FOR_READING
     */
    the_rwlock->number_of_readers += 1;
    the_rwlock->current_state = CORE_RWLOCK_LOCKED_FOR_READING;

    /*
     * Now see if more readers can be let go.
     */
    while ( 1 ) {
      next = _Thread_queue_First(
        &the_rwlock->Wait_queue,
        CORE_RWLOCK_TQ_OPERATIONS
      );
      if ( !next ||
           next->Wait.option == CORE_RWLOCK_THREAD_WAITING_FOR_WRITE )
        return CORE_RWLOCK_SUCCESSFUL;
      the_rwlock->number_of_readers += 1;
      _Thread_queue_Extract( next );
    }
  }

  /* indentation is to match _ISR_Disable at top */

  return CORE_RWLOCK_SUCCESSFUL;
}